Lithium containing glass or glass ceramic article with modified K2O profile near the glass surface

ABSTRACT

A method of reworking lithium containing ion exchanged glass articles is provided. The method includes a reverse ion exchange process that returns the glass article to approximately the composition of the glass from which the glass article was produced, before being subjected to ion exchange. The reworked glass articles exhibit a K 2 O concentration profile comprising a portion wherein a K 2 O concentration increases to a local K 2 O concentration maximum.

This application is a divisional of U.S. application Ser. No. 15/750051filed Feb. 2, 2018, which is a national phase of International PatentApplication PCT/US2017/016249, filed on Feb. 2, 2017, the content ofeach of which are incorporated herein by reference in their entirety.

BACKGROUND

The disclosure relates to a chemically strengthened glass article. Moreparticularly, the disclosure relates to chemically strengthened glassarticles comprising a surface layer having a compressive stress and aK₂O concentration peak below the surface of the glass article.

Glass articles are widely used in electronic devices as cover plates orwindows for portable or mobile electronic communication andentertainment devices, such as cellular phones, smart phones, tablets,video players, information terminal (IT) devices, laptop computers andthe like, as well as in other applications. As glass articles becomemore widely used, it has become more important to develop strengthenedglass articles having improved survivability, especially when subjectedto tensile stresses and/or relatively deep flaws caused by contact withhard and/or sharp surfaces.

The chemical strengthening process may produce glass articles withsurface defects, such as scratches, or undesired stress profiles. Foreconomic reasons, it is desirable to rework glass articles with suchdefects to increase the yield of strengthened glass articles with thedesired characteristics. However, removing material from the surface ofstrengthened glass articles necessitates the re-ion exchange of thematerial to achieve the desired surface compressive stresscharacteristics, which can produce undesired dimensional changes orwarping of the glass article. Additionally, the re-ion exchange step mayproduce undesired internal diffusion of the ions introduced during thechemical strengthening procedure and relaxation of the stress in theglass article. Thus, there is a need for a process to increase the yieldof chemically strengthened articles by enabling the rework of chemicallystrengthened glass articles with an non-desired stress profile orsurface defects such that the resulting glass articles exhibit a desiredstress profile and surface compressive stress.

SUMMARY

The present disclosure provides in aspect (1) an alkali aluminosilicateglass article comprising: Li₂O, Na₂O, and K₂O; a compressive stresslayer extending from a surface of the alkali aluminosilicate glass to adepth of compression (DOC); a tensile region extending from the depth oflayer into the glass article and having a maximum tensile stress of atleast about 40 MPa; and a K₂O concentration profile comprising a portionwherein a K₂O concentration increases to a local K₂O concentrationmaximum.

In aspect (2) the alkali aluminosilicate glass article of aspect (1) isprovided, wherein the local K₂O concentration maximum is located at adepth in a range from about 3 μm to about 30 μm below the surface of thealkali aluminosilicate glass article.

In aspect (3) the alkali aluminosilicate glass article of aspects (1) or(2) is provided, wherein the local K₂O concentration maximum has a K₂Oconcentration of 0.05 mol % to 1.2 mol %.

In aspect (4) of the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the local K₂O concentrationmaximum has a K₂O concentration of 0.5% to 15% of the K₂O concentrationat the surface of the alkali aluminosilicate glass article.

In aspect (5) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the maximum tensile stress is atleast about 50 MPa.

In aspect (6) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein a maximum compressive stress ofthe compressive stress layer is at least about 600 MPa.

In aspect (7) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, further comprising from about 0.1 mol %to about 10 mol % B₂O₃.

In aspect (8) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the alkali aluminosilicate glassarticle is substantially free of B₂O₃.

In aspect (9) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the alkali aluminosilicate glassarticle comprises: from about 58 mol % to about 65 mol % SiO₂; fromabout 11 mol % to about 20 mol % Al₂O₃; from about 6 mol % to about 18mol % Na₂O; from 0 mol % to about 6 mol % MgO; from 0.1 mol % to about13 mol % Li₂O; and from 0 mol % to about 6 mol % ZnO.

In aspect (10) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the alkali aluminosilicate glassarticle comprises from about 0.5 mol % to about 2.8 mol % P₂O₅.

In aspect (11) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, further comprising a thickness in a rangefrom about 0.05 mm to about 1.5 mm.

In aspect (12) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, further comprising at least one ofsilver, copper, cesium, and rubidium.

In aspect (13) the alkali aluminosilicate glass article of any of thepreceding aspects is provided, wherein the alkali aluminosilicate glassarticle comprises a glass ceramic.

In aspect (14) a consumer electronic device is provided comprising: ahousing; electrical components provided at least partially internal tothe housing, the electrical components including at least a controller,a memory, and a display, the display being provided at or adjacent to afront surface of the housing; and a cover article disposed at or overthe front surface of the housing and over the display, wherein thehousing or cover article comprises the alkali aluminosilicate glassarticle of any of the preceding aspects.

In aspect (15) an alkali aluminosilicate glass article is providedcomprising: Li₂O, Na₂O, and K₂O; a compressive stress layer extendingfrom a surface of the alkali aluminosilicate glass to a depth ofcompression (DOC); a tensile region extending from the depth of layerinto the glass article and having a maximum tensile stress of at leastabout 40 MPa; and a K₂O concentration at a depth of about 15 μm to about25 μm that is at least about 0.3 mol % higher than a K₂O concentrationat a center of the alkali aluminosilicate glass article.

In aspect (16) the alkali aluminosilicate glass article of aspect (15)is provided, wherein the local K₂O concentration maximum has a K₂Oconcentration of 0.3 mol % to 1.2 mol %.

In aspect (17) the alkali aluminosilicate glass article of aspects (15)or (16) is provided, wherein a depth of about 15 μm to about 25 μmcomprises a K₂O concentration from 0.5% to 15% of the K₂O concentrationat the surface of the alkali aluminosilicate glass article.

In aspect (18) the alkali aluminosilicate glass article of any ofaspects (15) to (17) is provided, wherein a maximum compressive stressof the compressive stress layer is at least about 600 MPa.

In aspect (19) the alkali aluminosilicate glass article of any ofaspects (15) to (18) is provided, further comprising a thickness in arange from about 0.05 mm to about 1.5 mm.

In aspect (20) the alkali aluminosilicate glass article of any ofaspects (15) to (19) is provided, further comprising at least one ofsilver, copper, cesium, and rubidium.

In aspect (21) the alkali aluminosilicate glass article of any ofaspects (15) to (20) is provided, wherein the alkali aluminosilicateglass article comprises a glass ceramic.

In aspect (22) a method is provided comprising: reverse ion exchangingan ion exchanged glass article in a reverse ion exchange bath to producea reverse ion exchanged glass article, wherein the reverse ion exchangebath comprises a lithium salt; and re-ion exchanging the reverse ionexchanged glass article in a re-ion exchange bath to form a re-ionexchanged glass article, wherein the re-ion exchanged glass articlecomprises: Li₂O, Na₂O, and K₂O; a compressive stress layer extendingfrom a surface of the re-ion exchanged glass article to a depth ofcompression (DOC); a tensile region extending from the depth of layerinto the re-ion exchanged glass article and having a maximum tensilestress of at least about 40 MPa; and a K₂O concentration profilecomprising a portion wherein a K₂O concentration increases to a localK₂O concentration maximum.

In aspect (23) the method of aspect (22) is provided, furthercomprising: removing between 1 μm and 10 μm from the surface of thereverse ion exchanged glass article.

In aspect (24) the method of aspect (23) is provided, wherein theremoving comprises mechanical polishing or chemical etching.

In aspect (25) the method of any of aspects (22) to (24) is provided,wherein the reverse ion exchange bath comprises: 3 wt % to 40 wt %LiNO₃; and 55 wt % to 97 wt % NaNO₃.

In aspect (26) the method of any of aspects (22) to (25) is provided,wherein the reverse ion exchange bath comprises at most 1 wt % KNO₃.

In aspect (27) the method of any of aspects (22) to (26) is provided,wherein the reverse ion exchange bath is free of KNO₃.

In aspect (28) the method of any of aspects (22) to (27) is provided,wherein the reverse ion exchange bath is at a temperature of about 320°C. to about 520° C.

In aspect (29) the method of any of aspects (22) to (28) is provided,further comprising reverse ion exchanging the reverse ion exchangedglass article in a second reverse ion exchange bath, wherein the secondreverse ion exchange bath comprises a lithium salt.

In aspect (30) the method of aspect (29) is provided, wherein the secondreverse ion exchange bath comprises: 0.1 wt % to about 5.0 wt % LiNO₃;and NaNO₃.

In aspect (31) the method of aspects (29) or (30) is provided, whereinthe second reverse ion exchange bath is substantially free of KNO₃.

In aspect (32) the method of any of aspects (29) to (31) is provided,wherein the reverse ion exchange in the second reverse ion exchange bathextends for a period of about 5 to about 30 minutes.

In aspect (33) the method of any of aspects (29) to (32) is provided,wherein the second reverse ion exchange bath is at a temperature ofabout 320° C. to about 520° C.

In aspect (34) the method of any of aspects (22) to (33) is provided,wherein the reverse ion exchange in the reverse ion exchange bathextends for a period of about 2 hours to about 48 hours.

In aspect (35) the method of any of aspects (22) to (34) is provided,wherein the re-ion exchange bath comprises: about 15 wt % to about 40 wt% NaNO₃; and about 60 wt % to about 85 wt % KNO₃.

In aspect (36) the method of any of aspects (22) to (35) is provided,wherein the re-ion exchange in the re-ion exchange bath extends for aperiod of about 30 minutes to about 120 minutes.

In aspect (37) the method of any of aspects (22) to (36) is provided,wherein the re-ion exchange bath is at a temperature of about 350° C. toabout 420° C.

In aspect (38) the method of any of aspects (22) to (37) is provided,further comprising re-ion exchanging the re-ion exchanged glass articlein a second re-ion exchange bath.

In aspect (39) the method of aspect (38) is provided, wherein the secondre-ion exchange bath comprises: about 3 wt % to about 15 wt % NaNO₃; andabout 85 wt % to about 97 wt % KNO₃.

In aspect (40) the method of aspects (38) or (39) is provided, whereinthe second re-ion exchange bath is at a temperature of about 350° C. toabout 420° C.

In aspect (41) the method of any of aspects (38) to (40) is provided,wherein the re-ion exchange in the second re-ion exchange bath extendsfor a period of about 10 minutes to about 30 minutes.

In aspect (42) the method of any of aspects (22) to (41) is provided,further comprising ion exchanging a glass article in an ion exchangebath to form the ion exchanged glass article.

In aspect (43) the method of aspect (42) is provided, wherein the ionexchange bath comprises: about 15 wt % to about 40 wt % NaNO₃; and about60 wt % to about 85 wt % KNO₃.

In aspect (44) the method of aspects (42) or (43) is provided, whereinthe ion exchange in the ion exchange bath extends for a period of about30 minutes to about 120 minutes.

In aspect (45) the method of any of aspects (42) to (44) is provided,wherein the ion exchange bath is at a temperature of about 350° C. toabout 420° C.

In aspect (46) the method of any of aspects (42) to (45) is provided,further comprising ion exchanging the ion exchanged glass article in asecond ion exchange bath.

In aspect (47) the method of aspect (46), wherein the second ionexchange bath comprises: about 3 wt % to about 15 wt % NaNO₃; and about85 wt % to about 97 wt % KNO₃.

In aspect (48) the method of aspects (46) or (47) is provided, whereinthe second ion exchange bath is at a temperature of about 350° C. toabout 420° C.

In aspect (49) the method of any of aspects (46) to (48), wherein there-ion exchange in the second re-ion exchange bath extends for a periodof about 10 minutes to about 30 minutes.

In aspect (50) the method of any of aspects (22) to (49) is provided,wherein the re-ion exchanged glass article comprises: a Li₂Oconcentration at a depth of 10 μm below a surface of the re-ionexchanged glass article of about 0.5% to about 20% of the Li₂Oconcentration at the surface of the re-ion exchanged glass article; anda K₂O concentration at a depth of 10 μm below the surface of the re-ionexchanged glass article of about 0.5% to about 20% of the K₂Oconcentration at the surface of the re-ion exchanged glass article.

In aspect (51) a lithium containing glass article is providedcomprising: an intensity coupling profile including a plurality ofcoupling resonances; wherein a first of the coupling resonances has ahalf-width half-maximum value that is at least 1.8 times greater thanthe half-width half-maximum value of a second of the couplingresonances.

In aspect (52) the lithium containing glass article of aspect (51) isprovided, wherein the first of the coupling resonances has a half-widthhalf-maximum value that is at least 2 times greater than the half-widthhalf-maximum value of a second of the coupling resonances.

In aspect (53) the lithium containing glass article of aspects (51) or(52) is provided, wherein an intensity coupling profile of both thetransverse magnetic polarization and the transverse electronicpolarization comprises a first of the coupling resonances has ahalf-width half-maximum value that is at least 1.8 times greater thanthe half-width half-maximum value of a second of the couplingresonances.

In aspect (54) the lithium containing glass article of any of aspects(51) to (53) is provided, wherein a first of the coupling resonances hasa half-width half-maximum value that is at least 1.8 times greater thanthe half-width half-maximum value of a second of the coupling resonancesand a third of the coupling resonances.

In aspect (55) a lithium containing glass article is provided,comprising: a smoothed intensity coupling profile including a pluralityof coupling resonances; wherein a second derivative of the smoothedintensity coupling profile at a first of the coupling resonances is atleast 1.8 times greater than a second derivative of the smoothedcoupling profile at a second of the coupling resonances.

In aspect (56) the lithium containing glass article of aspect (55) isprovided, wherein the second derivative of the smoothed intensitycoupling profile at the first of the coupling resonances is at least 1.8times greater than the second derivative of the smoothed couplingprofile at the second of the coupling resonances.

In aspect (57) the lithium containing glass article of aspect (55) or(56) is provided, wherein an intensity coupling profile of both thetransverse magnetic polarization and the transverse electronicpolarization comprises a second derivative of the smoothed intensitycoupling profile at a first of the coupling resonances is at least 1.8times greater than a second derivative of the smoothed coupling profileat a second of the coupling resonances.

In aspect (58) the lithium containing glass article of any of aspects(55) to (57) is provided, wherein a second derivative of the smoothedintensity coupling profile at a first of the coupling resonances is atleast 1.8 times greater than a second derivative of the smoothedcoupling profile at a second of the coupling resonances and a third ofthe coupling resonances.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a strengthened alkalialuminosilicate glass article according to one or more embodiments;

FIG. 2 is a flow chart of a production process of an ion exchanged glassarticle;

FIG. 3 is a flow chart of a rework process including a reverse ionexchange process;

FIG. 4 is xLi_g as a function of xLi_b based on experimental data and afit to the experimental data;

FIG. 5 is schematic, front plan view of a consumer electronic productincluding one or more embodiments of the alkali aluminosilicate glassarticles described herein;

FIG. 6 is a schematic, prospective view of the consumer electronicproduct of FIG. 5;

FIG. 7 is a plot of oxide concentration as a function of depth in an ionexchanged glass article;

FIG. 8 is a plot of stress as a function of depth in an ion exchangedglass article and a polished and re-ion exchanged glass article;

FIG. 9 is a representation of stress as a function of depth for atheoretical ion exchanged glass;

FIG. 10 is percent weight gain for a glass article when allowed to reachequilibrium in a variety of LiNO₃ containing ion exchange baths;

FIG. 11 is K₂O concentration as a function of depth in a glass articleafter a dual ion exchange process and a variety of reverse ion exchangeprocesses;

FIG. 12 is the peak K₂O concentration as a function of reverse ionexchange time for a variety of reverse ion exchange baths;

FIG. 13 is Na₂O concentration as a function of depth in a glass articleafter a dual ion exchange process and a variety of reverse ion exchangeprocesses;

FIG. 14 is Li₂O concentration as a function of depth in a glass articleafter a dual ion exchange process and a variety of reverse ion exchangeprocesses;

FIG. 15 is the K₂O concentration as a function of depth in a glassarticle after various stages in a rework reverse ion exchange process;

FIG. 16 is the oxide concentrations as a function of depth in a glassarticle after various stages in a rework reverse ion exchange process;

FIG. 17 is the oxide concentrations as a function of depth in a glassarticle after a rework reverse ion exchange process;

FIG. 18 is the stress of ion exchanged and reverse ion exchanged glassarticles as a function of depth below a surface of the glass articles;

FIG. 19 is the K₂O concentration as a function of depth in a glassarticle for various reverse ion exchange processes;

FIG. 20 is a series of FSM spectra for glass articles at differentstages in a reverse ion exchange rework process;

FIG. 21 is a Weibull plot of edge strength as measure using a 4 pointbend test;

FIG. 22 is a FSM spectra for an ion exchanged glass article;

FIG. 23 is a FSM spectra for a reverse ion exchanged glass article;

FIG. 24 is the prism-coupling-intensity signal of FIG. 22;

FIG. 25 is the prism-coupling-intensity signal of FIG. 23;

FIG. 26 is the prism-coupling-intensity signal of a reverse ionexchanged glass article;

FIG. 27 is a plot of the first-derivative and the second-derivative ofFIG. 24;

FIG. 28 is a plot of the first-derivative and the second-derivative ofFIG. 25;

FIG. 29 is a plot of the first-derivative and the second-derivative ofFIG. 26.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. It also is understood that the various featuresdisclosed in the specification and the drawings can be used in any andall combinations.

As used herein, the term “glass article” is used in its broadest senseto include any object made wholly or partly of glass, including glassceramics. Unless otherwise specified, all compositions of the glassesdescribed herein are expressed in terms of mole percent (mol %), and theconstituents are provided on an oxide basis. Coefficients of thermalexpansion (CTE) are expressed in terms of parts per million (ppm)/° C.and represent a value measured over a temperature range from about 20°C. to about 300° C., unless otherwise specified. High temperature (orliquid) coefficients of thermal expansion (high temperature CTE) arealso expressed in terms of part per million (ppm) per degree Celsius(ppm/° C.), and represent a value measured in the high temperatureplateau region of the instantaneous coefficient of thermal expansion(CTE) vs. temperature curve. The high temperature CTE measures thevolume change associated with heating or cooling of the glass throughthe transformation region.

Unless otherwise specified, all temperatures are expressed in terms ofdegrees Celsius (° C.). As used herein the term “softening point” refersto the temperature at which the viscosity of a glass is approximately10⁷⁶ poise (P), the term “anneal point” refers to the temperature atwhich the viscosity of a glass is approximately 10^(13.2) poise, theterm “200 poise temperature (T^(200 P))” refers to the temperature atwhich the viscosity of a glass is approximately 200 poise, the term“10¹¹ poise temperature” refers to the temperature at which theviscosity of a glass is approximately 10¹¹ poise, the term “35 kPtemperature (T^(35 kP))” refers to the temperature at which theviscosity of a glass is approximately 35 kilopoise (kP), and the term“160 kP temperature)(T^(160 kP))” refers to the temperature at which theviscosity of a glass is approximately 160 kP.

As used herein, the term “zircon breakdown temperature” or“T^(breakdown)”, refers to the temperature at which zircon—which iscommonly used as a refractory material in glass processing andmanufacture—breaks down to form zirconia and silica, and the term“zircon breakdown viscosity” refers to the viscosity of the glass atT^(breakdown). The term “liquidus viscosity” refers to the viscosity ofa molten glass at the liquidus temperature, wherein the liquidustemperature refers to the temperature at which crystals first appear asa molten glass cools down from the melting temperature, or thetemperature at which the very last crystals melt away as temperature isincreased from room temperature. The term “35 kP temperature” or“T^(35 kP)” refers to the temperature at which the glass or glass melthas a viscosity of 35,000 Poise (P), or 35 kiloPoise (kP).

It is noted that the terms “substantially” and “about” may be utilizedherein to represent the inherent degree of uncertainty that may beattributed to any quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. For example, a glass that is “substantiallyfree of K₂O” is one in which K₂O is not actively added or batched intothe glass, but may be present in very small amounts as a contaminant.

Compressive stress (including surface CS) is measured by surface stressmeter (FSM) using commercially available instruments such as theFSM-6000, manufactured by Orihara Industrial Co., Ltd. (Japan). Surfacestress measurements rely upon the accurate measurement of the stressoptical coefficient (SOC), which is related to the birefringence of theglass. SOC in turn is measured according to Procedure C (Glass DiscMethod) described in ASTM standard C770-16, entitled “Standard TestMethod for Measurement of Glass Stress-Optical Coefficient,” thecontents of which are incorporated herein by reference in theirentirety. Maximum tensile stress or central tension (CT) values aremeasured using a scattered light polariscope (SCALP) technique known inthe art.

As used herein, depth of compression (DOC) means the depth at which thestress in the chemically strengthened alkali aluminosilicate glassarticle described herein changes from compressive to tensile. DOC may bemeasured by FSM or a scattered light polariscope (SCALP) depending onthe ion exchange treatment. Where the stress in the glass article isgenerated by exchanging potassium ions into the glass article, FSM isused to measure DOC. Where the stress is generated by exchanging sodiumions into the glass article, SCALP is used to measure DOC. Where thestress in the glass article is generated by exchanging both potassiumand sodium ions into the glass, the DOC is measured by SCALP, since itis believed the exchange depth of sodium indicates the DOC and theexchange depth of potassium ions indicates a change in the magnitude ofthe compressive stress (but not the change in stress from compressive totensile); the exchange depth of potassium ions in such glass articles ismeasured by FSM.

The depth of penetration of K+ ions (“Potassium DOL”) is distinguishedfrom DOC because it represents the depth of potassium penetration as aresult of an ion exchange process. The Potassium DOL is typically lessthan the DOC for the articles described herein. Potassium DOL ismeasured using a surface stress meter such as the commercially availableFSM-6000 surface stress meter, manufactured by Orihara Industrial Co.,Ltd. (Japan), which relies on accurate measurement of the stress opticalcoefficient (SOC), as described above with reference to the CSmeasurement.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

Described herein are methods for reworking chemically strengthened glassarticles which exhibit a manufacturing defect, and the resultingreworked glass articles. The defects may include surface defects orundesired stress profiles. Surface defects may be the result of handlingduring the manufacturing process, and may include scratches, dents, anddimples. An undesired stress profile may result from ion exchangeconditions that are outside of specifications.

Surface defects may be removed from a chemically strengthened glassarticle by removing material from the surface, such as by polishing oretching. The removal of material from the surface of the glass articlealso removes a portion of the glass article that is under compressivestress. Thus, the glass article must be subjected to an additional ionexchange to achieve the desired stress profile after removing materialfrom the surface. The additional ion exchange may negatively impact thestrength and dimensional stability of the glass article. For example,the additional ion exchange may produce internal diffusion and stressrelaxation in the glass article, as well as result in part growth thatrenders the glass article outside of desired dimensional tolerances.

The methods described herein include a reverse ion exchange step thatserves to return the chemically strengthened glass articles toapproximately the composition of the glass article prior to the chemicalstrengthening ion exchange. The reverse ion exchange step employs areverse ion exchange bath comprising a lithium salt and a sodium salt.After the reverse ion exchange the glass article may be optionallyprocessed to remove material from the surface before re-ion exchangingthe glass article to produce a desired stress profile. The reworkedglass articles contain a buried high index peak that corresponds to aK₂O concentration profile having a portion wherein the K₂O concentrationincreases to a local K₂O concentration maximum, allowing the reworkedglass articles to be distinguished from a non-reworked glass article.

An exemplary process for manufacturing a chemically strengthened glassarticle and determining whether parts require rework is illustrated inFIG. 2. As shown in FIG. 2, the glass article manufacturing process mayinclude score cutting a glass sheet formed by a fusion process,machining the edges and/or forming holes in the resulting parts,optionally 3-D forming the machined parts, and then optionally polishingthe edges and surface of the parts. The parts are then chemicallystrengthened in an ion exchange process to form ion exchanged glassarticles. The ion exchanged glass articles are then optionally polishedto remove less than 1 μm of material from each side of the ion exchangedglass articles before inspecting the glass articles to determine if theymeet manufacturing standards. The parts that do not meet the desiredstandards are then processed as rework to increase the yield of themanufacturing process. Parts may fail to meet manufacturing standardsfor a variety of reasons, such as including surface defects or having anundesired stress profile. Surface defects may be formed as a result ofhandling during various stages of the manufacturing process. Afterinspection the parts which were not designated for rework may have ananti-fingerprint coating and/or decoration applied. The parts are theninspected again to determine if they meet manufacturing standards, andthe parts that fail to meet the manufacturing standards are processed asrework.

FIG. 3 illustrates an exemplary rework processing method. In someembodiments, one or more of the steps shown in FIG. 3 are not performed.In some embodiments, additional steps not shown in FIG. 3 may beperformed as part of the rework processing method. The rework processingmethod includes reverse ion exchanging the ion exchanged glass articlesin a lithium salt containing reverse ion exchange bath to produce areverse ion exchanged glass article. If the ion exchanged articlesinclude a decoration, the decoration may be removed before the reverseion exchange. In some embodiments, anti-fingerprint (AF) coatings do notneed to be removed before the ion exchanged glass articles are subjectedto reverse ion exchange. The reverse ion exchanged glass articles may beoptionally subjected to mechanical polishing or chemical etching toremove material from the surface of the reverse ion exchanged glassarticles. Removing material from the surface of the reverse ionexchanged glass articles may also remove surface defects. The reverseion exchanged glass articles may then be re-ion exchanged in a re-ionexchange bath to form a re-ion exchanged glass article. The re-ionexchanged glass article is then inspected to determine whether the partfalls within the desired manufacturing standards. An anti-fingerprintcoating and/or a decoration may then be applied to the re-ion exchangedglass article before a final inspection to determine whether the partfalls within the desired manufacturing standards.

The reverse ion exchange process removes ions from the ion exchangedglass article to return the glass article to approximately its pre-ionexchange state. The composition of the reverse ion exchange bath isselected to remove the ions added to the glass article during the ionexchange process. In some embodiments, for example, but not limited to,when the non-ion-exchanged glass article comprises Li₂O and Na₂O, thereverse ion exchange bath may contain LiNO₃ and NaNO₃, with the relativeamounts of LiNO₃ and NaNO₃ selected such that equilibrium weight gain ofthe non-ion exchanged glass article in the reverse ion exchange bath isnear zero or positive. If the LiNO₃ content of the reverse ion exchangebath is too high, an excess of LiO₂ may accumulate in the surface of thereverse ion exchanged glass article, producing surface tension that mayproduce surface cracks in the glass articles. The reverse ion exchangebath may include about 3 wt % to about 40 wt % LiNO₃; such as 3 wt % toabout 33 wt % LiNO₃; about 5 wt % to about 30 wt % LiNO₃, about 10 wt %to about 25 wt % LiNO₃, about 15 wt % to about 20 wt % LiNO₃, or anysub-ranges contained therein or defined by any of these end points. Thereverse ion exchange bath may contain about 55 wt % to about 97 wt %NaNO₃; such as about 60 wt % to about 97 wt % NaNO₃; about 67 wt % toabout 97 wt % NaNO₃; about 70 wt % to about 95 wt % NaNO₃, about 75 wt %to about 90 wt % NaNO₃, about 80 wt % to about 85 wt % NaNO₃, or anysub-ranges contained therein or defined by any of these end points. Insome embodiments, up to about 5 wt % of the NaNO₃ in the reverse ionexchange bath may be replaced by KNO₃, such as in situations where thesame reverse ion exchange bath is employed for multiple rework processcycles and potassium removed from the glass article poisons the reverseion exchange bath. In some embodiments, the reverse ion exchange bathmay contain less than about 5 wt % KNO₃, less than about 1 wt % KNO₃,and may be free of KNO₃.

The reverse ion exchange bath composition may be determined based on thecomposition of the glass article before ion exchange. The appropriatereverse ion exchange bath composition may be determined based on thedesired Li₂O and Na₂O content in the glass article. In particular, xLi_gis the molar fraction of Li₂O with respect to the total amount of Li₂Oand Na₂O in the glass prior to ion exchange; and xLi_b is the molarfraction of LiNO₃ with respect to the total of LiNO₃ and NaNO₃ in thereverse ion exchange bath. As shown in FIG. 4, a sigmoidal curve was fitto the average-composition data obtained after interpreting theexperimental data of weight changes after ion-exchange presented in FIG.10. The data points on the curves represent preferable reverse ionexchange bath molar compositions in terms of fraction of Li with respectto the total Li and Na ions, for each target molar composition of theglass, xLi_g. In an example where the target glass composition afterreversal of chemical strengthening has xLi_g at about 0.36, thepreferred bath composition has xLi_b at about 0.2 (or, in terms ofweight, the bath has about 17 wt % LiNO₃ and about 83 wt % NaNO₃). Atthe same time, favorable results of repeated chemical strengthening wereobtained not only after reverse ion exchange in a bath having 17 wt %LiNO₃, but also after reverse ion exchange in a bath having about 12 wt% LiNO₃ and about 88 wt % NaNO₃. These results demonstrate that it ispossible to obtain chemical strengthening that falls within the producttarget specifications when the bath composition differs by as much as 5wt % (and similarly, about 5 mol % LiNO₃) from the optimum compositionwhen the latter is at about xLi_b=0.2, with target xLi_b approximatelyequal to 0.36. Given the changes in slope in the curve of FIG. 4, thebath compositions xLi_b recommended for a particular glass compositiontarget xLi_g may be bounded by maximum deviations from the optimumcomposition that equal about 3 mol % for 0≤xLi_g≤0.2, about 4 mol % for0.2≤xLi_g≤0.3, about 5 mol % for 0.3≤xLi_g≤0.4, about 6 mol % for0.4≤xLi_g≤0.8, about 5 mol % for 0.8≤xLi_g≤0.9, and about 4 mol % for0.9≤xLi_g≤1.0.

The fit in FIG. 4 is described by the following equation for x_(Li) ^(g)expressed in terms of x_(Li) ^(b):

${x_{Li}^{g} = {0.5\left( {1 \pm \sqrt{1 - \frac{R - 1}{{pR} - 1}}} \right)}},$where:

$p = {1 - {{\exp\left( \frac{ɛ}{kT} \right)}.}}$

In an example, T=693 K, kT=0.05973 eV, and

${\frac{ɛ}{kT} \approx {- 0.9}},$such that the interaction energy ε is negative, ε≈−0.0574 eV, andp≈0.5934. Such that:

${R = \left( \frac{K^{\prime} - 1}{K^{\prime} + 1} \right)^{2}},$where:

${K^{\prime} = {K_{eq}\frac{x_{Na}^{b}}{x_{Li}^{b}}}},$and where K_(eq)=0.61, and x_(Li) ^(b) and x_(Na) ^(b) are the molarconcentrations of LiNO₃ and NaNO₃ measured relative to the total amountof LiNO₃+NaNO₃. For the purposes of at least the lithium containingglass composition described in Example 1 below, the optimum compositionmay be depicted as the curve in FIG. 4, and is defined by the followingequations. The following equations are also generally applicable forlithium containing glass compositions similar to the glass compositiondescribed in Example 1.

For the present experiment conducted at 420° C., when the target Na inthe glass exceeds the target Li in the glass, x_(Li) ^(g)≤0.5:

$x_{Li}^{b} = {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}} \equiv {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1\mspace{14mu}\sqrt{\begin{matrix}{1 - {4x_{Li}^{g}x_{Na}^{g}}} \\{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}\end{matrix}}}}.}}$

When the target Li in the glass exceeds the target Na in the glass,x_(Li) ^(g)>0.5:

$x_{Li}^{b} = {\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}} \equiv {\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1\mspace{14mu} 2.374x_{Li}^{g}x_{Na}^{g}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}}.}}$

In the above, the following definitions are employed, the value of kT isexpressed in electronvolts (eV), and for the present example thesubstitution was made kT=0.05973 eV corresponding to the temperature of420° C.

The molar ratio xLi_b or x_(Li) ^(o) in a salt bath of LiNO₃ to thetotal amount of LiNO₃ and NaNO₃ is given by:

$x_{Li}^{b} - {\frac{\left\lbrack {LiNO}_{3} \right\rbrack}{\left\lbrack {LiNO}_{3} \right\rbrack + \left\lbrack {NaNO}_{3} \right\rbrack}.}$

The molar ratio xLi_g or x_(Li) ^(g) in a glass of Li₂O to the totalamount of Li₂O and Na₂O is given by:

$x_{Li}^{g} = {\frac{\left\lbrack {{Li}_{2}O^{g}} \right\rbrack}{\left\lbrack {{Li}_{2}O^{g}} \right\rbrack + \left\lbrack {{Nc}_{2}O^{g}} \right\rbrack}.}$

The molar ratio xNa_g or x_(Na) ^(g) in a glass of Na₂O to the totalamount of Na₂O and Li₂O is given by:

$x_{Na}^{g} = {\frac{\left\lbrack {{Na}_{2}O^{g}} \right\rbrack}{\left\lbrack {{Li}_{2}O^{g}} \right\rbrack + \left\lbrack {{Na}_{2}O^{g}} \right\rbrack}.}$

In addition to the above described the above recommended maximumdeviations of x_(Li) ^(g) from the optimum values described by theequilibrium curve of FIG. 4, a narrower range where x_(Li) ^(b) iswithin 2 mol % of the optimum value would be preferred, and in addition,in some cases it may be required that x_(Li) ^(b) be within 1 mol % ofthe optimum value given by the equilibrium curve and the equations thatdescribe it.

When 0.4≤xx_(Li) ^(g)≤0.5 the recommended range for x_(Li) ^(b) is:

${\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} - 0.06} < x_{Li}^{b} < {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} + {0.06.}}$

When 0.3≤x_(Li) ^(g)≤0.4 the recommended range for is:

${\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1\mspace{14mu}\sqrt{\begin{matrix}{1 - {4x_{Li}^{g}x_{Na}^{g}}} \\{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}\end{matrix}}}} - 0.05} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1\mspace{14mu}\sqrt{\begin{matrix}{1 - {4x_{Li}^{g}x_{Na}^{g}}} \\{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}\end{matrix}}}} + {0.05.}}$

When 0.2≤x_(Li) ^(g)≤0.3 the recommended range for x_(Li) ^(g) is:

${\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} - 0.04} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} + {0.04.}}$

In some embodiments, it may be preferred that these ranges for x_(Li)^(g) are narrower, such as ±0.02, and in some cases, ±0.01, of theoptimum value.

Similarly, when the target x_(Li) ^(g) is greater than 0.5, thefollowing are the recommended ranges for x_(Li) ^(b). When 0.5≤x_(Li)^(g)≤0.8, the recommended range for is:

${\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} - 0.06} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} + {0.06.}}$

When, 0.8≤x_(Li) ^(g)≤0.9, the recommended range for x_(Li) ^(b) is:

${\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1\mspace{14mu} 2.374x_{Li}^{g}x_{Na}^{g}}}}} - 0.05} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1\mspace{14mu} 2.374x_{Li}^{g}x_{Na}^{g}}}}} + {0.05.}}$

When 0.9≤x_(Li) ^(g)≤1.0, x_(Li) ^(b) is chosen to not exceed 1.0, andwithin the range:

${\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} - 0.04} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {2.374x_{Li}^{g}x_{Na}^{g}}}}}} + {0.04.}}$

In some embodiments, it may be preferred that these ranges for x_(Li)^(b) are narrower, such as ±0.02, and in some cases, ±0.01, of theoptimum value.

More generally, if the reverse ion exchange is performed at atemperature substantially different from 420° C., the molar compositionof the bath x_(Li) ^(b) is to be within about Δx_(Li) ^(b) of theoptimum molar composition, where Δx_(Li) ^(b) takes on one of the values0.06, 0.05, 0.04, 0.03, 0.02, or 0.01 as described above, and when thetarget glass composition is x_(Li) ^(g)>0.5:

${\frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}} - {\Delta x}_{Li}^{b}} \leq x_{Li}^{b} \leq \frac{0.61}{0.61 + \frac{1 - \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1❘\sqrt{\begin{matrix}{1 - {4x_{Li}^{g}x_{Na}^{g}}} \\{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}\end{matrix}}}}$

When the target glass composition is x_(Li) ^(g)≤0.5:

${\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}} - {\Delta x}_{Li}^{b}} \leq x_{Li}^{b} \leq {\frac{0.61}{0.61 + \frac{1 + \sqrt{\frac{1 - {4x_{Li}^{g}x_{Na}^{g}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}{1 - \sqrt{\frac{\;{1 - {4x_{Li}^{g}x_{Na}^{g}}}}{1 - {4\left( {1 - {\exp\left( {- \frac{0.0574}{kT}} \right)}} \right)x_{Li}^{g}x_{Na}^{g}}}}}} + {\Delta x}_{Li}^{b}}$

The reverse ion exchange process may include the reverse ion exchange ofthe ion exchanged glass article in the reverse ion exchange bath for atime of about 2 hours to about 48 hours; such as about 2 hours to about24 hours; about 2 hours to about 16 hours; about 4 hours to about 14hours, about 6 hours to about 12 hours, about 8 hours to about 10 hours,or any sub-ranges contained therein or defined by any of these endpoints. The reverse ion exchange bath may be at a temperature of about320° C. to about 520° C.; such as about 320° C. to about 450° C.; about380° C. to about 420° C., or any sub-ranges contained therein or definedby any of these end points.

In some embodiments, the rework process may include a second reverse ionexchange in a second reverse ion exchange bath. The second reverse ionexchange bath may contain more or less LiNO₃ than the first reverse ionexchange bath.

In some embodiments, for example embodiments where the sodium content ofthe glass article is depleted to a degree that is greater than desiredduring the first reverse ion exchange process, the second reverse ionexchange bath may include less LiNO₃ and more NaNO₃ than the firstreverse ion exchange bath. In such cases, the second reverse ionexchange bath may comprise less than about 5 wt % LiNO₃; such as lessthan about 4 wt % LiNO₃, less than about 3 wt % LiNO₃, less than about 2wt % LiNO₃, or any sub-ranges contained therein, with the balancecomprising NaNO₃ and a possible KNO₃ poisoning contribution. The secondreverse ion exchange may take place at a temperature of about 320° C. toabout 520° C.; such as about 320° C. to about 450° C.; about 380° C. toabout 420° C., or any sub-ranges contained therein or defined by any ofthese end points. The second reverse ion exchange may extend for aperiod of about 5 minutes to about 30 minutes.

In some embodiments, the second reverse ion exchange bath may containmore LiNO₃ than the first reverse ion exchange bath. Such a reverse ionexchange bath may be employed where the lithium content of the glassarticle before ion exchange would require a LiNO₃ content of the reverseion exchange bath that may produce cracking in the glass article ifperformed in a single reverse ion exchange process. Employing multiplereverse ion exchange baths allows the content of LiNO₃ in subsequentreverse ion exchange baths to be increased until the necessary LiNO₃content to reach the LiO₂ content of the original glass article isachieved without cracking the glass article. The temperature of and thetime for which the second, and potentially subsequent, reverse ionexchange bath extends may be within the parameters described above forthe reverse ion exchange.

The reverse ion exchanged glass article may then be subjected tomechanical polishing or chemical etching to remove material from thesurface of the glass article, and any surface defects present at thesurface of the reverse ion exchanged glass article. The amount ofmaterial removed from the reverse ion exchanged glass article may be ina range of about 1 μm to about 10 μm; such as about 3 μm to about 10 μm,about 5 μm to about 10 μm, or any sub-ranges contained therein. Thechemical etching process may be an acid etching process, such as ahydrofluoric acid etching process.

The reverse ion exchanged glass article may be re-ion exchanged in are-ion exchange bath to produce a re-ion exchanged glass article. There-ion exchange bath may be the same as the ion exchange bath employedto produce the ion exchanged glass article. Similarly, the re-ionexchanged glass article may be subjected to a second re-ion exchangeprocess in a second re-ion exchange bath. The second re-ion exchangebath may be the same as a second ion exchange bath employed to producethe ion exchanged glass article.

The re-ion exchanged glass article exhibits a buried high index peak.The buried high index peak may indicate the presence of a K₂Oconcentration profile having a portion wherein the K₂O concentrationincreases to a local K₂O concentration maximum in the re-ion exchangedglass article. The local K₂O concentration maximum may be located at adepth below a surface of the re-ion exchanged glass article of about 3μm to about 30 μm. The K₂O concentration at the local maximum may beabout 0.05 mol % to about 1.2 mol %, and may be about 0.5% to about 15%of the starting surface K₂O concentration of the ion exchanged glassarticle. The re-ion exchanged glass article may have a K₂O concentrationof about 0.5% to about 20% of the K₂O concentration of the ion exchangedglass article at a depth below a surface of the re-ion exchanged glassarticle of 10 μm. In some embodiments, the re-ion exchanged glassarticle may have a K₂O concentration at a depth below a surface of 10 μmof at least about 0.3 mol % greater than the K₂O concentration at thecenter of the re-ion exchanged glass article; such as 0.5 mol % greater;1 mol % greater; 1.5 mol % greater; 2 mol % greater, or more.

In one or more embodiments, the alkali aluminosilicate glass articleshave a homogenous microstructure (i.e., the glass is not phaseseparated). In one or more embodiments, the alkali aluminosilicate glassarticle is amorphous. As used herein, “amorphous” when used to describeglass article means substantially free of crystallites or crystallinephases (i.e., containing less than 1 vol % crystallites or crystallinephases).

The alkali aluminosilicate glass articles described herein may be formedfrom glass compositions that are fusion formable. In one or moreembodiments, the fusion formable glass composition may have a liquidusviscosity greater than about 200 kilopoise (kP) and, in someembodiments, having a liquidus viscosity of at least about 600 kP. Insome embodiments, these glass articles and compositions are compatiblewith a zircon isopipe: the viscosity at which the glass breaks down thezircon isopipe to create zirconia defects is less than 35 kP. Selectedglass compositions within the composition ranges described herein mayhave a zircon breakdown viscosity that is greater than 35 kP. In suchinstances, an alumina isopipe may be used to fusion form these glassarticles.

In one or more embodiments, the alkali aluminosilicate glass articlesmay include a glass composition that comprises at least 0.5 mol % P₂O₅,Na₂O and Li₂O. In some embodiments, Li₂O (mol %)/Na₂O (mol %) may beless than 1. In addition, these glasses may be free of B₂O₃ and K₂O. Thealkali aluminosilicate glasses described herein may further include ZnO,MgO, and SnO₂.

In some embodiments, the alkali aluminosilicate glass article comprisesor consists essentially of at least about 58 mol % SiO₂, from about 0.5mol % to about 3 mol % P₂O₅, at least about 11 mol % Al₂O₃, Na₂O andLi₂O.

In one or more embodiments, the alkali aluminosilicate glass articlecomprises or consists essentially of from about 58 mol % to about 65 mol% SiO₂; from about 11 mol % to about 20 mol % Al₂O₃; from about 0.5 mol% to about 3 mol % P₂O₅; from about 6 mol % to about 18 mol % Na₂O; fromabout 0.1 mol % to 10 mol % Li₂O; from 0 mol % to about 6 mol % MgO; andfrom 0 mol % to about 6 mol % ZnO. In certain embodiments, the glasscomprises or consists essentially of from about 63 mol % to about 65 mol% SiO₂; from 11 mol % to about 19 mol % Al₂O₃; from about 1 mol % toabout 3 mol % P₂O₅; from about 9 mol % to about 20 mol % Na₂O; fromabout 2 mol % to 10 mol % Li₂O; from 0 mol % to about 6 mol % MgO; andfrom 0 mol % to about 6 mol % ZnO.

In one or more embodiments, the alkali aluminosilicate glass articlecomprises the ratio R₂O (mol %)/Al₂O₃ (mol %) that is less than about 2(e.g., less than about 1.8, less than about 1.6, less than about 1.5, orless than about 1.4), where R₂O=Li₂O+Na₂O.

In one or more embodiments, the alkali aluminosilicate glass articlecomprises the relationship where the total amount of SiO₂ and P₂O₅ thatis greater than 65 mol % and less than 67 mol % (i.e., 65 mol %<SiO₂(mol %)+P₂O₅ (mol %)<67 mol %). For example, the total amount of SiO₂and P₂O₅ may be in a range from about 65.1 mol % to about 66.9 mol %,from about 65.2 mol % to about 66.8 mol %, from about 65.3 mol % toabout 66.7 mol %, from about 65.4 mol % to about 66.6 mol %, from about65.5 mol % to about 66.5 mol %, from about 65.6 mol % to about 66.4 mol%, from about 65.7 mol % to about 66.3 mol %, from about 65.8 mol % toabout 66.2 mol %, from about 65.9 mol % to about 66.1 mol %, or anysub-ranges contained therein or formed from any of these endpoints.

In one or more embodiments, the alkali aluminosilicate glass articlecomprises a relationship R₂O (mol %)+R′O (mol %)−Al₂O₃ (mol %)+P₂O₅ (mol%) that is greater than about −3 mol % (i.e., R₂O (mol %)+R′O (mol%)−Al₂O₃ (mol %)+P₂O₅ (mol %)>-3 mol %). In one or more embodiments, R₂Ois the total amount of Li₂O and Na₂O (i.e., R₂O=Li₂O+Na₂O). In one ormore embodiments, R′O is the total amount of divalent metal oxidespresent in the alkali aluminosilicate glass. In one or more embodiments,the relationship R₂O (mol %)+R′O (mol %)−Al₂O₃ (mol %)+P₂O₅ (mol %) thatis greater than about −2.5 mol %, greater than about −2 mol %, greaterthan about −1.5 mol %, greater than about −1 mol %, greater than about−0.5 mol %, greater than about 0 mol %, greater than about 0.5 mol %,greater than about 1 mol %, greater than about 1.5 mol %, greater thanabout 2 mol %, greater than about 2.5 mol %, greater than about 3 mol %,greater than about 3.5 mol %, greater than about 4 mol %, greater thanabout 4.5 mol %, greater than about 5 mol %, greater than about 5.5 mol%, or greater than about 6 mol %, greater than about 6.5 mol %, greaterthan about 7 mol %, greater than about 7.5 mol %, greater than about 8mol %, greater than about 8.5 mol %, greater than about 9 mol %, orgreater than about 9.5 mol %.

Each of the oxide components of the base (or unstrengthened) andstrengthened (i.e., chemically strengthened by ion exchange) alkalialuminosilicate glass articles described herein serves a function and/orhas an effect on the manufacturability and physical properties of theglass. Silica (SiO₂), for example, is the primary glass forming oxide,and forms the network backbone for the molten glass. Pure SiO₂ has a lowCTE and is alkali metal-free. Due to its extremely high meltingtemperature, however, pure SiO₂ is incompatible with the fusion drawprocess. The viscosity curve is also much too high to match with anycore glass in a laminate structure. In one or more embodiments, thealkali aluminosilicate glass article comprises SiO₂ in an amount in arange from about 58 mol % to about 65 mol %, from about 59 mol % toabout 64 mol %, from about 60 mol % to about 63 mol %, from about 61 mol% to about 62 mol %, from about 63.2 mol % to about 65 mol %, from about63.3 mol % to about 65 mol %, or any sub-ranges contained therein orformed from any of these endpoints.

In addition to silica, the alkali aluminosilicate glass articlesdescribed herein comprise the network former Al₂O₃ to achieve stableglass formation, low CTE, low Young's modulus, low shear modulus, and tofacilitate melting and forming. Like SiO₂, Al₂O₃ contributes to therigidity to the glass network. Alumina can exist in the glass in eitherfourfold or fivefold coordination, which increases the packing densityof the glass network and thus increases the compressive stress resultingfrom chemical strengthening. In one or more embodiments, the alkalialuminosilicate glass article comprises Al₂O₃ in an amount in a rangefrom about 11 mol % to about 20 mol %, from about 12 mol % to about 19mol %, from about 13 mol % to about 18 mol %, from about 14 mol % toabout 17 mol %, from about 15 mol % to about 16 mol %, or any sub-rangescontained therein or formed from any of these endpoints.

Phosphorous pentoxide (P₂O₅) is a network former incorporated in thealkali aluminosilicate glass articles described herein. P₂O₅ adopts aquasi-tetrahedral structure in the glass network; i.e., it iscoordinated with four oxygen atoms, but only three of which areconnected to the rest of the network. The fourth oxygen atom is aterminal oxygen that is doubly bound to the phosphorous cation. Theincorporation of P₂O₅ in the glass network is highly effective atreducing Young's modulus and shear modulus. Incorporating P₂O₅ in theglass network also reduces the high temperature CTE, increases theion-exchange interdiffusion rate, and improves glass compatibility withzircon refractory materials. In one or more embodiments, the alkalialuminosilicate glass article comprises P₂O₅ in an amount in a rangefrom about 0.5 mol % to about 5 mol %, from about 0.6 mol % to about 5mol %, from about 0.8 mol % to about 5 mol %, from about 1 mol % toabout 5 mol %, from about 1.2 mol % to about 5 mol %, from about 1.4 mol% to about 5 mol %, from about 1.5 mol % to about 5 mol %, from about1.6 mol % to about 5 mol %, from about 1.8 mol % to about 5 mol %, fromabout 2 mol % to about 5 mol %, from about 0.5 mol % to about 3 mol %,from about 0.5 mol % to about 2.8 mol %, from about 0.5 mol % to about2.6 mol %, from about 0.5 mol % to about 2.5 mol %, from about 0.5 mol %to about 2.4 mol %, from about 0.5 mol % to about 2.2 mol %, from about0.5 mol % to about 2 mol %, from about 2.5 mol % to about 5 mol %, fromabout 2.5 mol % to about 4 mol %, from about 2.5 mol % to about 3 mol %,or any sub-ranges contained therein or formed from any of theseendpoints.

The alkali aluminosilicate glass articles described herein describedherein may be free of B₂O₃, as its presence has a negative impact oncompressive stress when the glass is strengthened by ion exchange. Asused herein, the phrase “free of B₂O₃” means the alkali aluminosilicateglass articles described herein include less than about 0.1 mol % B₂O₃,less than about 0.05 mol % B₂O₃ or less than about 0.01 mol %. In someembodiments, the alkali aluminosilicate glass articles may include B₂O₃in an amount of about 0.1 mol % up to about 10 mol %; such as about 0.5mol % up to about 9 mol %; about 1 mol % up to about 8 mol %; about 2mol % up to about 7 mol %; about 3 mol % up to about 6 mol %; about 4mol % up to about 5 mol %, or any sub-ranges contained therein or formedfrom any of these endpoints.

The alkali oxide Na₂O is used to achieve chemical strengthening of thealkali aluminosilicate glass articles described herein by ion exchange.The alkali aluminosilicate glass articles described herein include Na₂O,which provides the Na+ cation to be exchanged for potassium cationspresent in a salt bath containing, for example, KNO₃. In someembodiments, the alkali aluminosilicate glass articles described hereincomprise from about 4 mol % to about 20 mol % Na₂O. In one or moreembodiments, the alkali aluminosilicate glass article comprises Na₂O inan amount in a range from about 4.5 mol % to about 19.5 mol %, fromabout 5 mol % to about 19 mol %, from about 5.5 mol % to about 18.5 mol%, from about 6 mol % to about 18 mol %, from about 6.5 mol % to about17.5 mol %, from about 7 mol % to about 17 mol %, from about 7.5 mol %to about 16.5 mol %, from about 8 mol % to about 16 mol %, from about8.5 mol % to about 15.5 mol %, from about 9 mol % to about 15 mol %,from about 9.5 mol % to about 14.5 mol %, from about 10 mol % to about14 mol %, from about 10.5 mol % to about 13.5 mol %, from about 11 mol %to about 13 mol %, from about 11.5 mol % to about 12.5 mol %, or anysub-ranges contained therein or formed from any of these endpoints.

The alkali aluminosilicate glass articles described herein contain Li₂O.In some embodiments, further include up to about 13 mol % Li₂O or up toabout 10 mol % Li₂O. In some embodiments, the alkali aluminosilicateglass articles comprise Li₂O in an amount in a range from about 0.1 mol% to about 10 mol %, from about 0.5 mol % to about 9.5 mol %, from about1 mol % to about 9 mol %, from about 1.5 mol % to about 8.5 mol %, fromabout 2 mol % to about 8 mol %, from about 2.5 mol % to about 7.5 mol %,from about 3 mol % to about 7 mol %, from about 3.5 mol % to about 6.5mol %, from about 4 mol % to about 6 mol %, from about 4.5 mol % toabout 5.5 mol %, or from about 4 mol % to about 8 mol %, or anysub-ranges contained therein or formed from any of these endpoints. Whensubstituted for Na₂O, Li₂O reduces the zircon breakdown temperature andsoftens the glass, which allows additional Al₂O₃ to be added to theglass. In the alkali aluminosilicate glass articles described herein,the amount of Na₂O present may exceed that of Li₂O, where Li₂O (mol%)/Na₂O (mol %)<1. In some embodiments, Li₂O (mol %)/Na₂O (mol %)<0.75.In some embodiments, R₂O (mol %)/Al₂O₃ (mol %)<2, and, in someembodiments, 0.9≤R₂O (mol %)/Al₂O₃ (mol %)≤1.6, where R₂O=Li₂O+Na₂O.

The presence of potassium oxide in the glass has a negative effect onthe ability of to achieve high levels of surface compressive stress inthe glass through ion exchange. The alkali aluminosilicate glassarticles described herein, as originally formed, may be free of K₂O. Inone or more embodiments, the alkali aluminosilicate glass articlesinclude less than about 0.2 mol % K₂O. However, when ion exchanged in apotassium-containing molten salt (e.g., containing KNO₃) bath, thealkali aluminosilicate glasses may include some amount of K₂O (i.e.,less than about 1 mol %), with the actual amount depending upon ionexchange conditions (e.g., potassium salt concentration in the ionexchange bath, bath temperature, ion exchange time, and the extent towhich K⁺ ions replace Li⁺ and Na⁺ ions). The resulting compressive layerwill contain potassium—the ion-exchanged layer near the surface of theglass may contain 10 mol % or more K₂O at the glass surface, while thebulk of the glass at depths greater than the depth of the compressivelayer remains essentially potassium-free.

In some embodiments, the alkali aluminosilicate glass articles describedherein may comprise from 0 mol % up to about 6 mol % ZnO, such as fromabout 0.5 mol % to about 5.5 mol %, from about 1 mol % to about 5 mol %,from about 1.5 mol % to about 4.5 mol %, from about 2 mol % to about 4mol %, from about 2.5 mol % to about 3.5 mol %, from about 0.1 mol % toabout 6 mol %, from about 0.1 mol % to about 3 mol %, or any sub-rangescontained therein or formed from any of these endpoints. The divalentoxide ZnO improves the melting behavior of the glass by reducing thetemperature at 200 poise viscosity (200P temperature). ZnO also isbeneficial in improving the strain point when compared to like additionsof P₂O₅, and/or Na₂O.

Alkaline earth oxides such as MgO and CaO, may also be substituted forZnO to achieve a similar effect on the 200P temperature and strainpoint. When compared to MgO and CaO, however, ZnO is less prone topromoting phase separation in the presence of P₂O₅. In some embodiments,the glasses described herein include from 0 mol % up to 6 mol % MgO or,in other embodiments, these glasses comprise from 0.02 mol % to about 6mol % MgO. While other alkaline earth oxides, including SrO and BaO, mayalso be substituted for ZnO, they are less effective in reducing themelt temperature at 200 poise viscosity than ZnO, MgO, or CaO and arealso less effective than ZnO, MgO, or CaO at increasing the strainpoint.

In some embodiments, the alkali aluminosilicate glass articles describedherein are formable by down-draw processes that are known in the art,such as slot-draw and fusion-draw processes. The lithium may be batchedin the melt as either spodumene or lithium carbonate.

The fusion draw process is an industrial technique that has been usedfor the large-scale manufacture of thin glass sheets. Compared to otherflat glass manufacturing techniques, such as the float or slot drawprocesses, the fusion draw process yields thin glass sheets withsuperior flatness and surface quality. As a result, the fusion drawprocess has become the dominant manufacturing technique in thefabrication of thin glass substrates for liquid crystal displays, aswell as for cover glass for personal electronic devices such asnotebooks, entertainment devices, tables, laptops, mobile phones, andthe like.

The fusion draw process involves the flow of molten glass over a troughknown as an “isopipe,” which is typically made of zircon or anotherrefractory material. The molten glass overflows the top of the isopipefrom both sides, meeting at the bottom of the isopipe to form a singlesheet where only the interior of the final sheet has made direct contactwith the isopipe. Since neither exposed surface of the final glass sheethas made contact with the isopipe material during the draw process, bothouter surfaces of the glass are of pristine quality and do not requiresubsequent finishing.

In order to be fusion drawable, a glass composition must have asufficiently high liquidus viscosity (i.e., the viscosity of a moltenglass at the liquidus temperature). In some embodiments, compositionsused to form the alkali aluminosilicate glass articles described hereinhave a liquidus viscosity of at least about 200 kilopoise (kP) and, inother embodiments, at least about 600 kP.

After the alkali aluminosilicate glass articles are formed, the articleis chemically strengthened. Ion exchange is widely used to chemicallystrengthen glasses. In one particular example, alkali cations within asource of such cations (e.g., a molten salt, or “ion exchange,” bath)are exchanged with smaller alkali cations within the glass to achieve alayer that is under a compressive stress near the surface of the glassarticle. The compressive layer extends from the surface to a DOC withinthe glass article. In the alkali aluminosilicate glass articlesdescribed herein, for example, potassium ions from the cation source areexchanged for sodium ions within the glass during ion exchange byimmersing the glass in a molten salt bath comprising a potassium saltsuch as, but not limited to, potassium nitrate (KNO₃). Other potassiumsalts that may be used in the ion exchange process include, but are notlimited to, potassium chloride (KCl), potassium sulfate (K₂SO₄),combinations thereof, and the like. The ion exchange baths describedherein may contain alkali ions other than potassium and thecorresponding salts. For example, the ion exchange bath may also includesodium salts such as sodium nitrate (NaNO₃), sodium sulfate, sodiumchloride, or the like. In one or more embodiments, a mixture of twodifferent salts may be utilized. For example, the glass articles may beimmersed in a salt bath of KNO₃ and NaNO₃. In some embodiments, morethan one bath may be used with the glass being immersed in one bathfollowed by another, successively. The baths may have the same ordifferent compositions, temperatures and/or may be used for differentimmersion times.

The ion exchange bath may have a temperature in the range from about320° C. to about 520° C., such as from about 320° C. to about 450° C.Immersion time in the bath may vary from about 15 minutes to about 48hours, such as from about 15 minutes to about 16 hours. In someembodiments, the ion exchange bath may comprise about 15 wt % to about40 wt % NaNO₃; and about 60 wt % to about 85 wt % KNO₃.

While the embodiment shown in FIG. 1 depicts a strengthened alkalialuminosilicate glass article 100 as a flat planar sheet or plate, thealkali aluminosilicate glass article may have other configurations, suchas three dimensional shapes or non-planar configurations. Thestrengthened alkali aluminosilicate glass article 100 has a firstsurface 110 and a second surface 112 defining a thickness t. In one ormore embodiments, (such as the embodiment shown in FIG. 1) thestrengthened alkali aluminosilicate glass article is a sheet includingfirst surface 110 and opposing second surface 112 defining thickness t.The strengthened alkali aluminosilicate glass article 100 has a firstcompressive layer 120 extending from first surface 110 to a depth oflayer d₁ into the bulk of the glass article 100. In the embodiment shownin FIG. 1, the strengthened alkali aluminosilicate glass article 100also has a second compressive layer 122 extending from second surface112 to a second depth of layer d₂. Glass article also has a centralregion 330 that extends from d₁ to d₂. Central region 130 is under atensile stress or central tension (CT), which balances or counteractsthe compressive stresses of layers 120 and 122. The depth d₁, d₂ offirst and second compressive layers 120, 122 protects the strengthenedalkali aluminosilicate glass article 100 from the propagation of flawsintroduced by sharp impact to first and second surfaces 110, 112 of thestrengthened alkali aluminosilicate glass article 100, while thecompressive stress minimizes the likelihood of a flaw penetratingthrough the depth d₁, d₂ of first and second compressive layers 120,122. DOC d1 and DOC d2 may be equal to one another or different from oneanother. In some embodiments, at least a portion of the central region(e.g., the portion extending from the DOC to a depth equal to 0.5 timesthe thickness of the article) may be free of K₂O (as defined herein).

The DOC may be described as a fraction of the thickness t. For example,in one or more embodiments, the DOC may be equal to or greater thanabout 0.1t, equal to or greater than about 0.11t, equal to or greaterthan about 0.12t, equal to or greater than about 0.13t, equal to orgreater than about 0.14t, equal to or greater than about 0.15t, equal toor greater than about 0.16t, equal to or greater than about 0.17t, equalto or greater than about 0.18t, equal to or greater than about 0.19t,equal to or greater than about 0.2t, equal to or greater than about0.21t. In some embodiments, the DOC may be in a range from about 0.08tto about 0.25t, from about 0.09t to about 0.24t, from about 0.10t toabout 0.23t, from about 0.11t to about 0.22t, from about 0.12t to about0.21t, from about 0.13t to about 0.20t, from about 0.14t to about 0.19t,from about 0.15t to about 0.18t, from about 0.16t to about 0.19t, or anysub-ranges contained therein or formed from any of these endpoints. Insome instances, the DOC may be about 20 μm or less. In one or moreembodiments, the DOC may be about 40 μm or greater, such as from about40 μm to about 300 μm, from about 50 μm to about 280 μm, from about 60μm to about 260 μm, from about 70 μm to about 240 μm, from about 80 μmto about 220 μm, from about 90 μm to about 200 μm, from about 100 μm toabout 190 μm, from about 110 μm to about 180 μm, from about 120 μm toabout 170 μm, from about 140 μm to about 160 μm, from about 150 μm toabout 300 μm, or any sub-ranges contained therein or formed from any ofthese endpoints.

In one or more embodiments, the strengthened alkali aluminosilicateglass article may have a maximum compressive stress (which may be foundat the surface or a depth within the glass article) of about 400 MPa orgreater, about 500 MPa or greater, about 600 MPa or greater, about 700MPa or greater, about 800 MPa or greater, about 900 MPa or greater,about 930 MPa or greater, about 1000 MPa or greater, or about 1050 MPaor greater.

In one or more embodiments, the strengthened alkali aluminosilicateglass article may have a maximum tensile stress or central tension (CT)of about 40 MPa or greater; such as about 45 MPa or greater, about 50MPa or greater, about 60 MPa or greater, about 70 MPa or greater, about75 MPa or greater, about 80 MPa or greater, or about 85 MPa or greater.In some embodiments, the maximum tensile stress or central tension (CT)may be in a range from about 40 MPa to about 100 MPa.

The alkali aluminosilicate glass articles described herein are, in someembodiments, ion-exchanged by immersion in a molten salt bath comprisingor consisting essentially of about 100% KNO₃ by weight (small amounts ofadditives such as silicic acid or the like may be added to the bath). Inorder to maximize the surface compressive stress, the glasses mayundergo a heat treatment followed by ion exchange.

In some embodiments, the ion exchanged glass articles may include largeions, such as silver, copper, cesium, or rubidium. The content of theselarge ions in the ion exchanged glass article may be up to about 5 mol%, such as up to about 3 mol %. In some embodiments, the re-ion exchangebath may include salts of the large ions, such that the large ions arere-ion exchanged into the reverse ion exchanged glass article as part ofthe rework process.

In some embodiments, the alkali aluminosilicate glass articles describedherein form a portion of a consumer electronic product, such as acellular phone or smart phone, laptop computers, tablets, or the like. Aschematic view of a consumer electronic product (e.g., a smart phone) isshown in FIGS. 5 and 6. A consumer electronic device 500 including ahousing 502 having front 504, back 506, and side surfaces 508;electrical components (not shown) that are at least partially inside orentirely within the housing and including at least a controller, amemory, and a display 510 at or adjacent to the front surface of thehousing; and a cover substrate 512 at or over the front surface of thehousing such that it is over the display. In some embodiments, the coversubstrate 512 may include any of the strengthened articles disclosedherein.

EXEMPLARY EMBODIMENTS Example 1

A 0.8 mm thick glass article having a composition of 64.13 mol % SiO₂,15.98 mol % Al₂O₃, 1.24 mol % P₂O₅, 6.41 mol % Li₂O, 10.86 mol % Na₂O,0.03 mol % K₂O, 1.17 mol % ZnO, 0.05 mol % SnO₂, 0.08 mol % MgO, 0.02mol % CaO, and 0.02 mol % Fe₂O₃ was ion exchanged in a dual ion exchangeprocess. The dual ion exchange process included a first and second ionexchange, where the first ion exchange included a bath containing 75 wt% NaNO₃ and 25 wt % KNO₃ at a temperature of 380° C. for 3 hours and 30minutes. The second ion exchange included a bath containing 5 wt % NaNO₃and 95 wt % KNO₃ at a temperature of 380° C. for 30 minutes. Theresulting concentrations of Li₂O, Na₂O, and K₂O as measured by glowdischarge optical emission spectroscopy (GDOES) as a function of depthbeneath a surface of the glass article are shown in FIG. 7.

The ion exchanged glass article was polished to remove 7 μm from thesurface thereof. The polished ion exchanged glass article was then ionexchanged in an ion exchange bath including 9 wt % NaNO₃ and 91 wt %KNO₃ to recover a desired surface compressive stress (CS) and depth ofcompression (DOC). The stress profiles of the ion exchanged glassarticle and the polished and ion exchanged glass article are shown inFIG. 8. The compressive stress at the knee of the stress profile (CSk)was reduced by about 50 MPa in the polished and ion exchanged glassarticle in comparison the ion exchanged glass article that was notpolished. The stress profile in FIG. 8 was measured by the Refractednear-field (RNF) method. When the RNF method is utilized to measure thestress profile, the maximum CT value provided by SCALP is utilized inthe RNF method. In particular, the stress profile measured by RNF isforce balanced and calibrated to the maximum CT value provided by aSCALP measurement. The RNF method is described in U.S. Pat. No.8,854,623, entitled “Systems and methods for measuring a profilecharacteristic of a glass sample”, which is incorporated herein byreference in its entirety. In particular, the RNF method includesplacing the glass article adjacent to a reference block, generating apolarization-switched light beam that is switched between orthogonalpolarizations at a rate of between 1 Hz and 50 Hz, measuring an amountof power in the polarization-switched light beam and generating apolarization-switched reference signal, wherein the measured amounts ofpower in each of the orthogonal polarizations are within 50% of eachother. The method further includes transmitting thepolarization-switched light beam through the glass sample and referenceblock for different depths into the glass sample, then relaying thetransmitted polarization-switched light beam to a signal photodetectorusing a relay optical system, with the signal photodetector generating apolarization-switched detector signal. The method also includes dividingthe detector signal by the reference signal to form a normalizeddetector signal and determining the profile characteristic of the glasssample from the normalized detector signal.

FIG. 9 further illustrates the concept of CSk. As shown in FIG. 9, CSkis the compressive stress at the transition between the “spike” portionof the stress profile and the deeper diffusion region of the stressprofile. The DOC and potassium DOL are also shown in FIG. 9, with thepotassium DOL labeled as DOL.

A non-ion exchanged glass article with the above described glasscomposition was allowed to reach equilibrium in a variety of reverse ionexchange baths containing LiNO₃. The weight gain as a percentage of theoriginal glass article weight was then calculated based on the ratio ofLiNO₃ to LiNO₃+NaNO₃ in the bath as shown in FIG. 10. The line fit tothe measured data indicates that an ideal reverse ion exchange bath thatexhibits no weight gain would contain about 17 wt % LiNO₃ and 83 wt %NaNO₃, as shown in FIG. 10.

The ion exchanged glass article was then subjected to a variety ofreverse ion exchange treatments. A reverse ion exchange bath including 5wt % LiNO₃ at 380° C. was employed for reverse ion exchange treatmentsof 2 hours, 4 hours and 8 hours. A reverse ion exchange bath including17 wt % LiNO₃ at 420° C. was employed for a reverse ion exchangetreatment of 8 hours. As shown in FIGS. 11 and 12, the K₂O concentrationdecreased with increasing reverse ion exchange time, and the 17 wt %LiNO₃ bath reduced the K₂O concentration more than the 5 wt % LiNO₃bath. The K₂O concentrations shown in FIGS. 11 and 12 were measuredusing the GDOES method. FIGS. 13 and 14 show the Na₂O concentrations andLi₂O concentrations in the samples. As demonstrated in FIGS. 13 and 14,the 17 wt % LiNO₃ reverse ion exchange bath produced Na₂O concentrationsand Li₂O concentrations that were approximately the same as the originalglass composition.

Example 2

The glass composition of Example 1 was formed into a glass article thatwas then processed with a dual ion exchange treatment. The dual ionexchange process included a first and second ion exchange, where thefirst ion exchange included a bath containing 36 wt % NaNO₃ and 64 wt %KNO₃ at a temperature of 380° C. for 80 minutes. The second ion exchangeprocess included a bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ at atemperature of 370° C. for 20 minutes. The ion exchanged glass articlewas then subjected to a reverse ion exchange process. The reverse ionexchange process included a bath containing 17 wt % LiNO₃ and 83 wt %KNO₃ at a temperature of 420° C. for 8 hours. A chemical etching processwas then employed to remove 4 μm per side of the glass article withhydrofluoric acid. The etched glass article was then re-ion exchanged ina bath containing 36 wt % NaNO₃ and 64 wt % KNO₃ at a temperature of380° C. for 80 minutes and a second ion exchange bath containing 5 wt %NaNO₃ and 95 wt % KNO₃ at a temperature of 370° C. for 20 minutes. TheK₂O concentration of the ion exchanged glass article is represented bythe solid line in FIG. 15, the K₂O concentration of the reverse ionexchanged glass article is represented by the dashed line in FIG. 15,and the K₂O concentration of the re-ion exchanged glass article isrepresented by the dot-dashed line in FIG. 15. The circled region inFIG. 15 highlights the additional K₂O concentration that results fromthe reverse ion exchange process. FIG. 16 shows the oxide concentrationsin the glass article as a function of depth, with the lines in FIG. 16corresponding to the same stages of the process as in FIG. 15.

Example 3

The glass composition of Example 1 was formed into a glass article witha thickness of 0.5 mm and processed with a dual ion exchange treatment.The dual ion exchange process included a first and second ion exchange,where the first ion exchange included a bath containing 36 wt % NaNO₃and 64 wt % KNO₃ at a temperature of 380° C. for 80 minutes. The secondion exchange included a bath containing 10 wt % NaNO₃ and 90 wt % KNO₃at a temperature of 370° C. for 20 minutes. The ion exchanged glassarticle was then subjected to a reverse ion exchange process. Thereverse ion exchange process included a bath containing 17 wt % LiNO₃and 83 wt % KNO₃ at a temperature of 420° C. for 8 hours. A chemicaletching process was then employed to remove 4 μm per side of the reverseion exchanged glass article with hydrofluoric acid. The etched glassarticle was then re-ion exchanged in a bath containing 36 wt % NaNO₃ and64 wt % KNO₃ at a temperature of 380° C. for 80 minutes and a second ionexchange bath containing 10 wt % NaNO₃ and 90 wt % KNO₃ at a temperatureof 370° C. for 20 minutes. The oxide concentration was measured byGDOES, as shown in FIG. 17. The additional K₂O concentration as a resultof the reverse ion exchange is highlighted by the dashed rectangle inFIG. 17.

Example 4

The glass composition of Example 1 was formed into a glass article andprocessed with a dual ion exchange treatment. The dual ion exchangeprocess included a first and second ion exchange, where the first ionexchange included a bath containing 36 wt % NaNO₃ and 64 wt % KNO₃ at atemperature of 380° C. for 80 minutes. The second ion exchange includeda bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ at a temperature of 370°C. for 20 minutes. Surface scratches and other defects were then removedfrom the ion exchanged glass article by removing 3 μm from the surfaceper side of the glass article. The glass article was then subjected to areverse ion exchange process. The reverse ion exchange process includeda bath containing 17 wt % LiNO₃ and 83 wt % KNO₃ at a temperature of420° C. for 8 hours. The reverse ion exchanged glass article was thenprocessed with a dual ion exchange treatment. The dual ion exchangeprocess included a first and second ion exchange, where the first ionexchange included a bath containing 36 wt % NaNO₃ and 64 wt % KNO₃ at atemperature of 380° C. for 80 minutes. The second ion exchange includeda bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ at a temperature of 370°C. for 20 minutes. The depth of layer of potassium (DOL), depth ofcompression (DOC), compressive stress (CS), compressive stress knee(CSk), and central tension (CT) for the ion exchanged glass articlebefore removing material from the surface and the re-ion exchanged glassarticle are reported in Table 1.

TABLE 1 DOL (μm) DOC (μm) CS (MPa) CSk (MPa) CT (MPa) Ion 8.1 100.0881.3 97.8 63.8 Exchanged Re-Ion 9.1 98.4 874.3 83.1 61.3 Exchanged

Example 5

The glass composition of Example 1 was formed into glass articles with athickness of 0.5 mm and processed with a dual ion exchange treatment.The dual ion exchange process included a first and second ion exchange,where the first ion exchange included a bath containing 36 wt % NaNO₃and 64 wt % KNO₃ at a temperature of 380° C. for 80 minutes. The secondion exchange included a bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ ata temperature of 370° C. for 20 minutes. The ion exchanged glassarticles were then subjected to a reverse ion exchange treatment. Onereverse ion exchange treatment included a bath containing 17 wt % LiNO₃and 83 wt % NaNO₃ at a temperature of 420° C. for 8 hours. Anotherreverse ion exchange treatment included a bath containing 17 wt % LiNO₃and 83 wt % NaNO₃ at a temperature of 420° C. for 16 hours. Anotherreverse ion exchange treatment included a bath containing 12 wt % LiNO₃and 88 wt % NaNO₃ at a temperature of 420° C. for 8 hours. Anotherreverse ion exchange treatment included a bath containing 12 wt % LiNO₃and 88 wt % NaNO₃ at a temperature of 420° C. for 16 hours. The reverseion exchanged glass articles were then subjected to the dual ionexchange treatment again. The resulting smoothed stress profiles of theglass articles as measured by RNF are similar, as shown in FIG. 18.

Example 6

The glass composition of Example 1 was formed into glass articles with athickness of 0.5 mm and processed with a dual ion exchange treatment.The dual ion exchange process included a first and second ion exchange,where the first ion exchange included a bath containing 36 wt % NaNO₃and 64 wt % KNO₃ at a temperature of 380° C. for 80 minutes. The secondion exchange included a bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ ata temperature of 370° C. for 20 minutes. The ion exchanged glassarticles were then subjected to reverse ion exchange treatments. Onereverse ion exchange treatment included a bath containing 17 wt % LiNO₃and 83 wt % NaNO₃ at a temperature of 420° C. for 8 hours. Anotherreverse ion exchange treatment included a bath containing 16 wt % LiNO₃,0.2 wt % KNO₃, and 83.8 wt % NaNO₃ at a temperature of 420° C. for 8hours. Another reverse ion exchange treatment included a bath containing15 wt % LiNO₃, 0.4 wt % KNO₃, and 84.6 wt % NaNO₃ at a temperature of420° C. for 8 hours. The K₂O concentration as measured by GDOES afterthe reverse ion exchange process as a function of depth below the glassarticle surface is shown in FIG. 19. The reverse ion exchanged glassarticles were then subjected to the dual ion exchange process again. FSMspectra of the ion exchanged glass articles, the reverse ion exchangedglass articles, and the re-ion exchanged glass articles are shown inFIG. 20. The dense fringes in the FSM spectra for the re-ion exchangedglass articles (after DIOX) shown in FIG. 20 are correlated to theadditional K₂O concentration present in the glass articles as a resultof the reverse ion exchange.

Example 7

The glass composition of Example 1 was formed into glass articles with athickness of 0.5 mm and a machined edge profile. The glass articles wereprocessed with a dual ion exchange treatment. The dual ion exchangeprocess included a first and second ion exchange, where the first ionexchange included a bath containing 36 wt % NaNO₃ and 64 wt % KNO₃ at atemperature of 380° C. for 80 minutes. The second ion exchange includeda bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ at a temperature of 370°C. for 20 minutes. One of the glass articles was then subjected to areverse ion exchange treatment including a bath containing 17 wt % LiNO₃and 83 wt % NaNO₃ at a temperature of 420° C. for 8 hours. The reverseion exchanged glass article was then acid etched with hydrofluoric acidto remove 3 μm from the surface of the glass article. The acid etchedglass article was then subjected to the dual ion exchange treatment as are-ion exchange treatment. The failure stress at which 10% of the glassarticles are expected to fail (B10) was determined, as shown in FIG. 21.The failure stress was measured using a 4 point bend test. The controlshown in FIG. 21 is a glass article after the dual ion exchangetreatment, but before the reverse ion exchange. The B10 value may behigher for the re-ion exchanged glass article due to an increase in edgestrength associated with acid etching to reduce surface flaws.

Example 8

The glass composition of Example 1 was formed into glass articles with athickness of 0.5 mm and a machined edge profile. The glass articles wereprocessed with a dual ion exchange treatment. The dual ion exchangeprocess included a first and second ion exchange, where the first ionexchange included a bath containing 36 wt % NaNO₃ and 64 wt % KNO₃ at atemperature of 380° C. for 80 minutes. The second ion exchange includeda bath containing 5 wt % NaNO₃ and 95 wt % KNO₃ at a temperature of 370°C. for 20 minutes. One of the glass articles was then subjected to areverse ion exchange treatment including a bath containing 17 wt % LiNO₃and 83 wt % NaNO₃ at a temperature of 420° C. for 8 hours. The reverseion exchanged glass article was then processed to remove 3 μm from thesurface of the glass article. The surface processed glass article wasthen subjected to the dual ion exchange treatment as a re-ion exchangetreatment.

Prism-coupling reflectance spectra were obtained using theprism-coupling surface stress meter FSM-6000, where the ion-exchangedglass substrate was optically coupled to the prism using an interfacingoil with refractive index of 1.72 at 595 nm, the same index as theprism. The prism-coupling reflectance spectra for the non-reverse ionexchanged glass article is shown in FIG. 22, with the upper half of FIG.22 corresponding to the transverse magnetic (TM) polarization. Theprism-coupling reflectance spectra for the reverse ion exchanged glassarticle is shown in FIG. 23, with the upper half of FIG. 23corresponding to the TM polarization.

The prism-coupling-intensity signals of the glass articles were thenobtained from the prism-coupling reflectance spectra using the methodsdescribed in U.S. Pat. No. 9,140,543 entitled “Systems and methods formeasuring the stress profile of ion-exchanged glass,” which isincorporated herein by reference in its entirety. The signals of about250 rows of pixels from the sensor area assigned to the transversemagnetic (TM) polarization were summed up to form a single signal whereeach point corresponds to the average of the corresponding column ofabout 250 pixels. This helps improve the signal-to-noise ratio, whencompared to taking the data from a single row of pixels. Theprism-coupling-intensity signal for the non-reverse ion exchanged glassarticle is shown in FIG. 24, and the prism-coupling-intensity signal forthe reverse ion exchanged glass article is shown in FIG. 25. Theprism-coupling-intensity signals may alternatively be calculated basedon the transverse electronic (TE) polarization, shown in the lower halfof FIG. 22, with the expectation of similar results.

As shown in FIG. 24, the prism-coupling-intensity signal includes threedips in the in the signal in the region of total internal reflection,with each dip having a breadth of from about 30 to 40 pixels measured asthe full-width at half-maximum, as commonly understood in the art. Thesedips indicate coupling resonances and may correspond to the K₂O presentin the K₂O spike at the surface of the ion exchanged glass articles.FIG. 25 demonstrates that the reverse ion exchanged glass articleexhibits additional narrower dips in the signal that occur close to thetransition from total internal reflection to partial reflection that arenot present for the non-reverse ion exchanged glass articles. Theadditional narrow dips in FIG. 25 correlate to the residual K₂Oconcentration present in the reverse ion exchanged glass article that isnot present in the non-reverse ion exchanged glass article. Withoutwishing to be bound by any particular theory, the dips associated withthe residual K₂O concentration may be narrower than the other dips inthe signal because the residual K₂O concentration is present at a deeperdepth than the K₂O in the spike located at the surface of the glassarticles, producing narrower coupling references. The narrow dips shownin FIG. 25 have a breadth of about 10 pixels, as measured by thefull-width half-maximum method. Thus, the broad dips of FIG. 25 have abreadth that is about 3.5 times greater than the narrow dips.

As shown in FIG. 25, the narrower dips in the signal have a breadth ofat least about 3.5 times lower than the breadth of the dips in theregion of total internal reflection. For purposes of confirmation, theprism-coupling-intensity signal of a glass article with the glasscomposition of Example 1 was subjected to a single step ion exchange inan ion exchange bath of 36 wt % NaNO₃ and 64 wt % KNO₃ at a temperatureof 380° C. for 100 minutes, a reverse ion exchange in a reverse ionexchange bath of 17 wt % LiNO₃ and 83 wt % NaNO₃ at a temperature of420° C. for 8 hours, and then re-ion exchanged under the same conditionsas the single step ion exchange. The prism-coupling-intensity signal ofthis additional example is shown in FIG. 26. As shown in FIG. 26, thebroad dips in the total internal reflectance region have a breadth ofabout 20 pixels and the narrow dips have a breadth of about 10 pixels,as determined by the full-width half-maximum method. Thus, the broaddips have a breadth that is about 2 times greater than the narrow dips.Based on FIGS. 25 and 26, the reverse ion exchanged glass articleprism-coupling-intensity signal has a first coupling resonance breadththat is at least 1.8 times greater than a second coupling resonance inthe prism-coupling-intensity signal; such as at least 2 times greater,or at least 3.5 times greater.

The breadth of the dips in the prism-coupling-intensity signal may bemeasured by alternative methods where the full-width at half-maximummethod is not possible, such as asymmetric coupling resonances. In somecases, the breadths of the coupling resonances may be measured ashalf-width at half-maximum. In another case, the breadth may be measuredat the 20% change in intensity measured from the darkest point of thecoupling resonance relative to the entire difference between the darkestpoint and the reference (pedestal) intensity.

Alternatively, the magnitude of the second derivative of theprism-coupling-intensity signal may be used to distinguish the reverseion exchanged glass articles from non-reverse ion exchanged glassarticles. As commonly practiced in the art, the prism-coupling-intensitysignal was conditioned by a modest low-pass filtering to reduce thehigh-frequency noise. In the present example the LOESS algorithm (W. S.Cleveland, “Robust Locally Weighted Regression And SmoothingScatterplots”, Journal of the American Statistical Association, Vol. 74,No. 368 (December 1979), p. 829-836) was employed to smooth theprism-coupling-intensity signal. Other low-pass filtering approaches maybe applied with a similar effect, as long as the underlying signal isnot significantly distorted and the coupling resonances are notsubstantially broadened. The prism-coupling-intensity signal was alsohistogram-shifted to improve the contrast by subtracting the lowestvalue of the signal from the intensity at every point, and the processedsignal was then rescaled to cover the intensity range 0-256. Thehistogram shift does not affect the relationship between the derivativesof different modes. In the present example, the image width is 1272columns of pixels.

Another commonly used method may be utilized, such as a moderatehigh-pass filter or band-pass filter, to remove a slowly-varyingcomponent of the prism-coupling-intensity signal resulting from aslowly-varying angular distribution of the illumination intensity. Thisadditional filtering was not necessary in the present example, but maybe useful to reduce distortion (causing asymmetry) of the broadercoupling resonances which are more significantly affected by such slowvariation of the angular distribution of illumination intensity. Anexemplary high-pass filter would have a spatial frequency cutoff that isin the range from 1/L up to about 5/L, where L is the length of thesignal, e.g., the number of columns of pixels. In some cases, thespatial frequency cutoff of high-pass filtering may be as high as 1/dL,where dL is the spacing in pixel columns between the two farthest-spacedneighboring coupling resonances.

FIG. 27 shows the first derivative and the second derivative of theprism-coupling-intensity signal from FIG. 24 processed as describedabove. In this example, the maxima of the second derivative at thelocations of the coupling resonances (pixel column positions 616, 893,and 1086) are all within a factor of 2.6 of each other. It is typicalfor the coupling resonances of non-reverse ion exchanged glass articlesto exhibit a local maximum of the second derivative that is within afactor of 3 of each other. The local maximums of the second derivativeare located at the zero-crossing of the first-derivative, whichindicates the location of a coupling resonance. Noise-inducedzero-crossings may be present in the first-derivative, and suchnoise-induced zero-crossings are easily rejected by software settingscommonly practiced in the art, such as minimum requirements for localintensity contrast, a minimum requirement for the local secondderivative of the signal, a requirement for a minimum spacing to thenext zero-crossing of the first derivative, and also comparison oflocations of neighboring detected zero-crossings to determine if theyfollow a sequence of that is typical of optical modes that arephysically possible. It is also easy for an operator to manuallyidentify the coupling resonances in the prism-coupling-intensity signal,and reject any noise-induced false positives. Noise-inducedzero-crossings are generally too closely spaced, and do not allowsignificant excursion of the derivatives over a large number of pixels,providing for easy rejection of such noise-induced false-positives.

FIG. 28 shows the first and second derivatives of the intensity signalfrom FIG. 25 processed as described above. The coupling resonances areidentified at pixel column numbers 657, 930, 1093, 1146 and 1159, withany false-positives being rejected by software settings. While the firstthree coupling resonances have a maximum second derivative that iswithin a factor of 3.4 of each other (1.67, 2.59, and 5.59,respectively). The last two coupling resonances have second derivativemaxima at 76.9 and 49.9, respectively. Thus, the last two couplingresonances have a second derivative maximum that ranges from 8.9 to 46times the maximum second derivative of any of the first three couplingresonances.

FIG. 29 shows the first and second derivatives of the intensity signalfrom FIG. 26 processed as described above. The coupling resonances arelocated at pixel columns 838, 1023, 1124, 1137, and 1150, rounded to thenearest column for simplicity. The second-derivative maxima at the firstand second coupling resonances are approximately equal at 13.5 and 14.0,respectively. The second-derivative maxima of the last 3 couplingresonances are 88.7, 71.1, and 42.5, respectively. Thus, the last 3coupling resonances have a second-derivative maxima that ranges from 3to 6.6 times the second-derivative maxima of any of the first twocoupling resonances. Additionally, relative to the first couplingresonance, shown on the left of FIG. 29, the second derivative maxima ofthe last 3 coupling resonances are at least 3.15 to 6.6 times larger.

In conducting the analysis of these examples, glass articles that weresubstantially free of warp were selected, to avoid broadening of thecoupling resonances due to warp. Additionally, the index fluid utilizedin the measurements should have a refractive index between therefractive index of the glass article and the refractive index of theprism, preferably closer to the refractive index of the prism. Forexample, if the glass article has a refractive index in the range1.49-1.52, and the prism index is 1.72, the preferred range forrefractive index of the index fluid would be 1.6-1.72. Optical lightblocks may be employed to reveal the broad coupling resonances withadequate contrast, as known in the state-of-the art.

The second-derivative maxima were measured without interpolation.Alternatively, a more precise comparison is possible if the signal isinterpolated on a denser array of data points, such as 5 data points perpixel, and then low-pass-filtered, since the second-derivative maximamay occur between pixels. In such a case, the peak values of the secondderivatives for the narrowest coupling resonances will be higher, andcloser to their true values. The broader coupling resonances would notbe substantially affected by the limitations of the measurement sensor,because they are usually sampled by many more pixels per couplingresonance linewidth.

From these examples, it is demonstrated that when there are at least 3broad coupling resonances that have second-derivative maxima within afactor of 3.5 of each other, the reverse ion exchanged glass articlesexhibit at least one coupling resonance with a second-derivative maximathat is more than a factor of 4 larger than that of the first couplingresonance, such as more than a factor of 5 greater, a factor of 6greater, a factor of 9 greater, or higher.

Further, when there are only 2 broad coupling resonances having asecond-derivative maxima within a factor of 2 of each other, the reverseion exchanged glass articles exhibit at least one coupling resonancewhose second derivative is at least a factor of 3 higher than the lowestsecond derivative maxima of any of the coupling resonances, such as afactor of 4 higher, a factor of 5 higher, a factor of 6 higher, orgreater.

In an alternative measurement process, the prism-coupling measurementmay be obtained at a longer wavelength, such as 780 nm. In some cases,the resolution of the prism-coupling measurement may not be adequate dueto close spacing of the narrow coupling resonances, or due to moderatewarp in the glass article. The measurement at a longer wavelength suchas 780 nm has reduced sensitivity to warp, and also somewhat largerspacing of the coupling resonances corresponding to a largereffective-index spacing of the guided optical modes. While the samecriteria for relative widths or relative magnitudes of the secondderivative of the two types of resonances can be applied at 780 nm, thelow-order modes that have the higher effective index and the broadercoupling resonances tend to couple more strongly at 780 nm, so a moreconservative criterion may be chosen if desired. In particular, if ameasurement at 595 nm is not possible or is questionable due tosignificant warpage, a reverse ion exchanged glass article as describedherein may exhibit at least one coupling resonance at 780 nm that has ahalf-width at half-maximum at least 1.8 times narrower than that ofanother coupling resonance in the same polarization state. In a specificembodiment, the wider coupling resonance is that of the lowest-ordermodes having the highest effective index. In a more conservativecriterion, a reverse ion exchanged glass article as described herein mayexhibit at least one coupling resonance at 780 nm that has a half-widthat half-maximum at least 2.5 times narrower than that of anothercoupling resonance in the same polarization state, such as at least 3times narrower, at least 4 times narrower, or more. In a specificembodiment, the wider coupling resonance is that of the lowest-ordermode having the highest effective index.

Similarly, if a measurement at 595 nm is not possible or is questionabledue to significant warpage, a reverse ion exchanged glass article asdescribed herein may exhibit at least one coupling resonance at 780 nmthat has a peak second derivative at least 3 times higher than that ofanother coupling resonance in the same polarization state. In a moreconservative criterion, a reverse ion exchanged glass article asdescribed herein may exhibit at least one coupling resonance at 780 nmthat has a peak second derivative that is at least 3.5 times higher thanthe peak second derivative of another coupling resonance in the samepolarization state, such as at least 4 time higher, at least 5 timeshigher, at least 6 times higher, at least 8 times higher, or more. In aspecific embodiment, the wider coupling resonance having the lower valueof its maximum second-derivative is that of the lowest-order mode havingthe highest effective index.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

The invention claimed is:
 1. An alkali aluminosilicate glass articlecomprising: Li₂O, Na₂O, and K₂O; a compressive stress layer extendingfrom a surface of the alkali aluminosilicate glass to a depth ofcompression (DOC); a tensile region extending from the depth of layerinto the glass article and having a maximum tensile stress of at leastabout 40 MPa; a potassium depth of layer (DOL), wherein the potassiumDOL is less than the DOC; and a K₂O concentration at a depth of about 15μm to about 25 μm that is at least about 0.3 mol % higher than a K₂Oconcentration at a center of the alkali aluminosilicate glass article.2. The alkali aluminosilicate glass article of claim 1, wherein a depthof about 15 μm to about 25 μm comprises a K₂O concentration that is from0.5% to 15% of a K₂O concentration at the surface of the alkalialuminosilicate glass article.
 3. The alkali aluminosilicate glassarticle of claim 1, wherein a maximum compressive stress of thecompressive stress layer is at least about 600 MPa.
 4. The alkalialuminosilicate glass article of claim 1, further comprising a thicknessin a range from about 0.05 mm to about 1.5 mm.
 5. The alkalialuminosilicate glass article of claim 1, further comprising at leastone of silver, copper, cesium, and rubidium.
 6. The alkalialuminosilicate glass article of claim 1, wherein the alkalialuminosilicate glass article comprises a glass ceramic.
 7. A consumerelectronic device comprising: a housing; electrical components providedat least partially internal to the housing, the electrical componentsincluding at least a controller, a memory, and a display, the displaybeing provided at or adjacent to a front surface of the housing; and acover article disposed at or over the front surface of the housing andover the display, wherein the housing or cover article comprises thealkali aluminosilicate glass article of claim 1.